The present invention relates to an optical sensor system employing an optical interface circuit. More particularly, the optical interface circuit provides an improved data signal for the validation and/or discrimination of color documents.
The use of optics to provide security checks and to validate color documents such as paper currency are known. Optical sensor systems typically comprise one or more light sources, a light detector, interface circuitry and a discriminator or microprocessor. The sensors measure the amount of light transmitted through or reflected from documents and an analog-to-digital converter outputs the digital data to a microprocessor for processing. Some prior art optical detection systems use an analog-to-digital converter to convert sampled optical signals into one of 256 possible values for processing. Measurements are typically taken over many predetermined places of the document under test.
U.S. Pat. No. 4,947,441 describes a bill discriminating apparatus having two color detectors for photoelectrically detecting light components contained in light transmitted through or reflected from bills to be discriminated. The bill discriminating circuitry includes a color correction circuit, two amplifying means, a gain adjustment circuit, and a differential amplifier to compare the outputs of the color detectors. A discriminator determines the validity of bills based on difference signals provided by an output from the differential amplifier.
A problem plaguing such prior art systems is that the electrical signal produced by the optical detectors typically comprise offsets which skew the bill data that is to be processed. The electrical offsets may include offsets resulting from manufacturing of the sensor, production of the optical system, and ambient light. Another electrical offset may be caused by an increase in temperature which produces a decrease in the illumination output of an LED light source. The corresponding decrease in detected light further skews the bill data.
These electrical offsets may be present in any combination and may vary from sensor to sensor. The combined electrical offsets make it virtually impossible for a microprocessor to discriminate actual bill data from the offsets. As a consequence, accurate ratio testing between two or more sensor outputs required by particular known acceptance algorithms is very difficult.
Several known calibration techniques are able to remove specific types of electrical offsets. For instance, one calibration technique utilizes additional hardware to regulate the current through LED light sources for adjusting their light output to balance the outputs of the corresponding optical sensors. This technique minimizes the electrical offsets due to manufacturing of the sensor and production of the system, but is costly to implement. Another well known calibration technique relies on the microprocessor to apply a stored compensation factor to the digital data obtained from the analog-to-digital converter to minimize the effects of the electrical offsets. However, this technique suffers from poor data resolution due to the inclusion of all of the electrical offsets in the data signal.
Therefore, a need exists for a low cost optical sensor system having an optical interface circuit that minimizes the effects of electrical offsets to achieve a maximum data resolution.
According to a first aspect of the invention, the optical sensor system comprises a processing unit connected to a controllable light source and the optical interface circuit. The controllable light source and the optical interface circuit are positioned relative to one another such that a document may be transported between them. A photodetector sensor and a photodetector interface circuit are disposed within the optical interface circuit. The photodetector interface circuit is connected to an output of the photodetector and to an input of the processing unit. The processing unit is capable of activating or deactivating the photodetector. When activated, the photodetector generates an electrical current signal at an output based on the amount of detected light. The photodetector interface circuit converts the generated electrical current to a signal that is provided to the processing unit. In generating the converted signal, the photodetector interface circuit compensates for electrical offsets, such as sensor manufacturing offsets, optical system production offsets and ambient light offsets, in the electrical current signal.
According to an embodiment of the present invention, the photodetector interface circuit may consist of a variable bias load, such as a capacitor and a reset switch, and a signal converter having an input and an output. The bias load and reset switch are connected to the photodetector output and to the signal converter input. The signal converter output is connected to the processing unit. The desired load resistance is achieved by resetting the capacitor to a reference voltage, such as zero volts, and then permitting it to charge in a linear manner for a charge time interval. The signal converter generates a digital signal based on the voltage across the capacitor at the end of the charge time interval. The load resistance may be varied by varying the charge time interval. The system reduces the effects of electrical offsets by adjusting the charge time interval accordingly.
According to another embodiment of the present invention, the photodetector interface circuit consists of an analog signal converter, a logarithmic current-to-voltage (I-to-V) converter, such as a diode, and a charge storage device. An input of the analog signal converter is connected to the photodetector output. An analog signal converter output is connected to the I-to-V converter and to the charge storage device. In this embodiment, the charge storage device is charged to a reference level based on the voltage drop over the I-to-V converter caused by a calibration signal at the analog signal converter input that corresponds to a maximum signal that can possibly be generated by the photodetector. Such charge is maintained while the photodetector output provides a signal to the analog signal converter based on the amount of detected light. The corresponding signal generated by the analog signal converter causes a voltage over the I-to-V converter which produces a difference voltage at one end of the charge storage device. The difference voltage corresponds to the difference of the calibration signal and the signal generated by the photodetector. The difference voltage is inversely logarithmically proportional to the amount of detected light with electrical offsets being substantially eliminated.
According to a further embodiment of the present invention, the processing unit measures the time interval required for the capacitor to charge to a predetermined voltage threshold in order to determine a document""s validity. In this embodiment, electrical offsets may be reduced by adjusting the corresponding threshold voltage or by scaling by reference compensation factors stored in the processing unit. According to yet another embodiment of the present invention, at least one photodetector interface circuit may be employed with a plurality of photodetectors, wherein two or more photodetectors are multiplexed to a corresponding photodetector interface circuit which facilitates the testing of various areas of a document with a reduced number of components.
Additional features and advantages of the present invention will become apparent by reference to the following detailed description and accompanying drawings.